With the progress of miniaturization and layer-increasing of semiconductor devises, polishing technologies such as Double Sided Polishing (DSP) have been an essential technology for production process of semiconductor devises.
In DSP for flattening, one of the important specification is in-plane uniformity (flatness) of finished thickness of a substrate. To improve the in-plane uniformity of finished thickness, it is important to control the finished thickness accurately. Accordingly, polishing apparatuses with a sizing device have been used for accurately monitoring the thickness of wafer in course of polishing (e.g., see Patent Document 1).
With increasing requirement for the in-plane uniformity (flatness) of a substrate in finished thickness, the sizing comes to be required to have accuracy of about ±0.1 μm or less, or even higher accuracy in recent years.
The devices that have been used for controlling the finished thickness include an eddy current sizing device, a sizing device to measure the distance to the upper face of carrier, and a sizing device using laser beam interference.
By the sizing apparatus to measure the distance to the upper face of carrier, however, the required sizing accuracy of about ±0.1 μm cannot be secured. When the eddy current sizing device is compared with the laser interferometric sizing device, the latter laser interferometric sizing device is superior in view of limitation of setting environment and measuring accuracy.
Accordingly, the laser interferometric sizing device has come to be widespread. The laser interferometric sizing device has become an essential technology particularly for highly accurate processing of a P− or P+ substrate. That is, the laser interferometric sizing device has become an essential technology to improve the uniformity of finished thickness in polishing such as DSP. The resistivity is commonly 10 Ω·cm or more in P− substrate, more than 0.01 Ω·cm and less than 10 Ω·cm in P+ substrate, and 0.01 Ω·cm or less in P++ substrate, particularly in this description.
In recent years, it becomes necessary to cope with high flatness also in the P++ substrate with lower specific resistance, and the laser interferometric sizing device has been reexamined. Hereinafter, the mechanism of laser interferometric sizing device will be described with exemplifying a double-side polishing apparatus.
In a laser interferometric sizing device, laser beam for interference pierces a hole formed to penetrate the turn table of the double-side polishing apparatus. In polishing of a substrate, the substrate is rotated and revolved by rotation of a gear that is engaged with a carrier, and the foregoing hole is formed on a position where the orbits of rotation and revolving of the substrate pass. Accordingly, the laser interferometric sizing device allows laser beam to pierce through this hole to irradiate the substrate in course of polishing with the laser beam, and allows the reflected light from the front and back surfaces of the substrate to be introduced into a light-receiving portion almost simultaneously.
These signals are introduced as digital signals and are recognized as information of the thickness of the substrate by using a Fourier transformation. In this case, the signals of the reflected light from the back surface of substrate can be introduced sufficiently from P− and P+ substrates, but the reflected light becomes weak in P++ substrates.
FIG. 6 shows a relation between resistivity of wafer (thickness of 775 μm) and transmittance of laser beam. The laser beam used for conventional sizing device has a wavelength of about 1300 nm, and the transmittance at this wavelength is about 50% in P− substrate, but only about 1% in P++ substrate.
Accordingly, measurement of the thickness of P++ substrate can be achieved by increasing the output of laser beam to about twice as much as in measuring the thickness of P− substrate and by setting the optimal region of the frequency. As described above, highly accurate sizing has become possible with the laser interferometric sizing device by changing the wavelength and intensity of laser beam.
In this way, it becomes possible to obtain thickness data not only in P− substrates and P+ substrates but also in P++ substrates, which have lower resistivity, by optimizing output of laser beam and signals used for a Fourier transformation.